Meeting the challenge of increasing worldwide demand for energy requires strategies for more efficient energy use as well as sustainable approaches to energy generation and conservation. Lighting is a major contributor to electricity consumption, accounting for 19% of global use and 34% for the U.S. The U.S. lighting market is currently divided among various lamp types as follows: 63% incandescent, 35% fluorescent, 2% halogen. The incandescent light bulb is only 5% efficient (15 lm/W) while the fluorescent lamp has 15-25% efficiency (50-80 lm/W).
Solid-state luminaires, which are typically based on light-emitting diodes (LEDs), have the potential to revolutionize the lighting industry with higher efficiency, better quality and lower maintenance, possibly leading to a reduction by half of energy consumed by general illumination. For example, 30% efficiency (100 lm/W at 350 mA) has been achieved in a commercially available white LED and 50% (150 lm/W at 20 mA) in a laboratory white LED device. The appreciable energy savings that come with converting from incandescent to fluorescent lamps (including compact fluorescent lamps or CFLs) and solid-state lighting has motivated many national governments to pass regulations that phase out the use of incandescent lights.
Traditional incandescent light sources are favored by many people because they provide a broad spectrum of incoherent light that produces a pleasing white light. A given LED, on the other hand, emits light of a specific color determined by the bandgap of the semiconductor material constituting the LED. One approach for producing white light is to use multiple LEDs of different colors—red (R), green (G), and blue (B).
Another approach is to use phosphors to transform blue or near-UV light from an LED, e.g., a GaN-based LED, to “pump” a phosphor or mixture of phosphors. The multiple LED approach leads to narrow spectral lines and limited in practice by the low efficiency of green LEDs. On the other hand, conventional blue LEDs coated with yellow phosphors give a cold white light, and are not color tunable. Some have used alternate sources for wavelength conversion with LEDs. An example wavelength conversion LED strategy is disclosed in Kim, US Patent Publication No. 2008/0185604, published on Aug. 7, 2008 Kim discloses a light emitting diode that emits in an ultraviolet, blue or green wavelength range. Linear homogenous nanowire or linear core shell nanowire phosphors convert light from the diode into a longer wavelength.
Another problem with solid state lighting based upon traditional light emitting diode structures is that the traditional solid state LEDs don't fit the power model that has been implemented for incandescent lighting. The power grid is a design that has a high voltage but low current power supply philosophy. Residential and business electrical services typically is defined by 110V or 220V service in the United States, and similar conventions are used in other parts of the world. An ideal suitable replacement for conventional general lighting would serve as a replacement for incandescent bulbs and meet the voltage and current limitations of standard residential or business services without need to resort to transformers requiring large step ups in either current or voltage. Compact fluorescent bulbs are a successful commercial product that meets this need, but many find their spectrum to be less pleasing than traditional incandescent sources. The compact fluorescent lamps also pose environmental and health risks because they contain mercury.
Traditional cathode ray tubes and similar devices use electrons to stimulate conventional phosphors, typically transition metal or rare earth compounds. The conventional phosphors limit efficiency because the phosphors must dissipate charge by emitting a photon before accepting additional energy.